STATEMENT OF GOVERNMENT INTEREST
[0001] The present invention was made, at least in part, with support from United States
Air Force contract number FA8650-04-2-2410. The Government may have certain rights
in this invention.
FIELD OF THE INVENTION
[0002] The present invention relates to the fabrication of semiconductor devices and more
particularly, to the fabrication of oxide layers on silicon carbide (SiC).
BACKGROUND
[0003] Silicon carbide (SiC) has a combination of electrical and physical properties that
make it attractive for a semiconductor material for high temperature, high voltage,
high frequency and high power electronic devices. These properties include a 3.0 eV
bandgap, a 4 MV/cm electric field breakdown, a 4.9 W/cm-K thermal conductivity, and
a 2.0x10
7 cm/s electron drift velocity.
[0004] Silicon carbide has the unique property among wide bandgap compound semiconductor
materials that it forms a native oxide. Thus, a thermal SiO
2 layer may be formed on a SiC layer. The ability to form a thermal oxide on SiC opens
the door for the formation of metal-oxide-semiconductor (MOS) devices using silicon
carbide, including, for example, MOS field-effect transistors (MOSFETs), MOS capacitors,
insulated gate bipolar transistors (IGBT's), MOS-controlled thyristors (MCTs), lateral
diffused MOSFETs (LDMOSFETs) and other related devices. Given the unique material
properties of SiC described above, such devices may have substantially better theoretical
operating characteristics compared to devices formed using other semiconductor materials,
particularly for applications requiring high power, high current capacity, and/or
high frequency operation. Accordingly, taking full advantage of SiC's electronic properties
in MOS devices and resulting integrated circuits requires appropriate SiC oxidation
technology.
[0005] The interface quality of SiO
2 thermally grown on a silicon substrate may be excellent. However, the quality of
thermally grown SiC/SiO
2 interfaces has not achieved the same levels as that of Si/SiO2 interfaces. Accordingly,
the quality of oxides on silicon carbide (SiC) has been a major obstacle to developing
commercially viable silicon carbide MOS devices. Indeed, with the recent improvements
in SiC crystal quality, oxide quality may perhaps be the largest barrier to the realization
of commercially viable SiC MOS power devices and integrated circuits.
[0006] Oxides on SiC have been widely reported to have unacceptably high densities of interface
states (or "traps") and fixed oxide charges, both of which may have an adverse effect
on MOS device performance. As used herein, the term "state" or "trap" refers to an
available energy level position within the bandgap of a semiconductor or insulator
material. An interface trap or state may be located at or near a semiconductor/insulator
interface. Interface states may occur due to the presence of dangling or unterminated
atomic bonds within a material. Thus, the density of electronic states at an interface
may be an indication of the amount of crystallographic disorder at the interface.
[0007] Interface traps may capture electronic charge carriers (i.e. electrons and/or holes),
which may produce undesired operating characteristics in devices incorporating the
interface. In particular, electronic states present at the SiC/SiO
2 interface may reduce surface electron mobility in the SiC layer. If the gate oxide
of a MOS device has a high density of interface states, the resulting device may have
reduced inversion channel mobility, increased threshold voltage, increased on-resistance
and/or other undesirable characteristics.
[0008] Recently, annealing of a thermal oxide in a nitric oxide (NO) ambient has shown promise
in a planar 4H-SiC MOSFET structure not requiring a p-well implant.
See M. K. Das, L. A. Lipkin, J. W. Palmour, G. Y. Chung, J. R. Williams, K. McDonald,
and L. C. Feldman, "High Mobility 4H-SiC Inversion Mode MOSFETs Using Thermally Grown,
NO Annealed SiO2," IEEE Device Research Conference, Denver, CO, June 19-21, 2000 and
G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, R. A. Weller, S. T. Pantelides,
L. C. Feldman, M. K. Das, and J. W. Palmour, "Improved Inversion Channel Mobility
for 4H-SiC MOSFETs Following High Temperature Anneals in Nitric Oxide," IEEE Electron
Device Letters accepted for publication, the disclosures of which are incorporated by reference
as if set forth fully herein. This anneal is shown to significantly reduce the interface
state density near the conduction band edge, as described in
G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, M. Di Ventra, S. T. Pantelides,
L. C. Feldman, and R. A. Weller, "Effect of nitric oxide annealing on the interface
trap densities near the band edges in the 4H polytype of silicon carbide," Applied
Physics Letters, Vol. 76, No. 13, pp. 1713-1715, March 2000, the disclosure of which is incorporated herein as if set forth fully. High electron
mobility (35-95 cm
2/Vs) is obtained in the surface inversion layer due to the improved MOS interface.
[0009] Unfortunately, NO is a health hazard having a National Fire Protection Association
(NFPA) health danger rating of 3, and the equipment in which post-oxidation anneals
are typically performed is open to the atmosphere of the cleanroom. They are often
exhausted, but the danger of exceeding a safe level of NO contamination in the room
is not negligible.
[0010] Growing the oxide in N
2O is possible as described in
J. P. Xu, P. T. Lai, C. L. Chan, B. Li, and Y. C. Cheng, "Improved Performance and
Reliability of N2O-Grown Oxynitride on 6H-SiC," IEEE Electron Device Letters, Vol.
21, No. 6, pp. 298-300, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein.
Xu et al. describe oxidizing SiC at 1100 °C for 360 minutes in a pure N
2O ambient and annealing in N
2 for 1 hour at 1100 °C.
[0011] Post-growth nitridation of the oxide on 6H-SiC in N
2O at a temperature of 1100 °C has also been investigated by
P. T. Lai, Supratic Chakraborty, C. L. Chan, and Y. C. Cheng, "Effects of nitridation
and annealing on interface properties of thermally oxidized SiO2/SiC metal-oxide-semiconductor
system," Applied Physics Letters, Vol. 76, No. 25, pp. 3744-3746, June 2000 (hereinafter, "Lai et al."), the disclosure of which is incorporated by reference
as if set forth fully herein. However, Lai
et al. concluded that such treatment deteriorates the interface quality which may be improved
with a subsequent wet or dry anneal in O
2 which may repair the damage induced by nitridation in N
2O. Moreover, even with a subsequent O
2 anneal, Lai
et al. did not see any significant reduction in interface state density as compared to the
case without nitridation in N
2O.
[0012] In addition to NO and N
2O growth and annealing, research has also been conducted on post growth anneals in
other environments. For example, Suzuki
et al. investigated post oxidation annealing in hydrogen.
Suzuki et al., "Effect of Post-oxidation-annealing in Hydrogen on SiO2/4H-SiC Interface,"
Material Science Forum, Vols. 338-342, pp. 1073-1076, 2000. These researchers reported that flatband voltage shift and interface state density
could be improved by post oxidation annealing in both argon and hydrogen. In this
research, 4H-SiC was oxidized in dry O
2 at 1200 °C. Post oxidation annealing was then carried out in argon or hydrogen for
30 minutes at 400, 700, 800 and 1000 °C. Other researchers, however, have reported
that post oxidation anneals in hydrogen provide no increased benefit over post oxidation
anneals in other gases, as described in
Mrinal Das, "Fundamental Studies of the Silicon Carbide MOS Structure," Doctoral Thesis,
Purdue University, submitted December, 1999.
SUMMARY
[0013] Embodiments of the present invention provide methods of forming oxide layers on silicon
carbide layers, including placing a silicon carbide layer in a chamber such as an
oxidation furnace tube that is substantially free of metallic impurities; heating
an atmosphere of the chamber to a temperature of about 500 °C to about 1300 °C; introducing
atomic oxygen in the chamber; and flowing the atomic oxygen over a surface of the
silicon carbide layer to thereby form an oxide layer on the silicon carbide layer.
In some embodiments, introducing atomic oxygen includes providing a source oxide in
the chamber and flowing a mixture of nitrogen and oxygen gas over the source oxide.
The source oxide may include aluminum oxide or another oxide such as manganese oxide.
[0014] In some embodiments, the source oxide is substantially free of metallic impurities.
For example, the source oxide may include a porous sapphire wafer. In such case, some
embodiments according to the invention include implanting a sapphire wafer with non-metallic
impurities to form a porous sapphire wafer, and placing the porous sapphire wafer
in the chamber.
[0015] In some embodiments, introducing atomic oxygen includes generating atomic oxygen
using a catalyst such as platinum. In some embodiments, introducing atomic oxygen
includes generating a flow of ozone and cracking the ozone to produce atomic oxygen.
Ozone may be cracked using thermal and/or electromagnetic energy.
[0016] In some embodiments, atomic oxygen may be generated outside the chamber and supplied
to the chamber.
[0017] In particular embodiments, the atmosphere of the chamber may be heated to a temperature
of about 1000 °C to 1100 °C.
[0018] Methods of forming an oxide layer on a silicon carbide layer according to some embodiments
of the invention include placing a silicon carbide layer in an oxidation chamber,
placing an alumina wafer in the chamber, heating an atmosphere of the chamber to a
temperature of about 500 °C to about 1300 °C, flowing nitrogen gas over the alumina
wafer to generate atomic oxygen in the chamber, and flowing the atomic oxygen over
a surface of the silicon carbide layer to thereby form an oxide layer on the silicon
carbide layer. In particular embodiments, the atmosphere of the chamber may be heated
to a temperature of about 1000 °C to 1100 °C. Further, oxygen gas may be flowed over
the alumina wafer. In such case, methods according to embodiments of the invention
may further include reacting the atomic oxygen with oxygen gas to produce ozone, flowing
the ozone over the alumina wafer, and cracking the ozone to produce atomic oxygen
in the vicinity of the silicon carbide layer.
[0019] Still further embodiments according to the invention include placing a silicon carbide
layer in a chamber, placing an alumina wafer in the chamber, heating an atmosphere
of the chamber to a temperature of about 500 °C to about 1300 °C, nitriding the alumina
wafer to liberate atomic oxygen, and flowing the atomic oxygen over a surface of the
silicon carbide layer to thereby form an oxide layer on the silicon carbide layer.
In particular further embodiments, the atmosphere of the chamber may be heated to
a temperature of about 1000 °C to 1100 °C. Further, oxygen gas may be flowed over
the alumina wafer. In such case, methods according to embodiments of the invention
may further include reacting the atomic oxygen with oxygen gas to produce ozone, flowing
the ozone over the alumina wafer, and cracking the ozone to produce atomic oxygen
in the vicinity of the silicon carbide layer.
[0020] In some embodiments, since source oxide wafers may be oriented in a vertical direction
parallel to the orientation of the silicon carbide layers such that a substantially
uniform distance between the source oxide wafers and the SiC layers is provided, which
may result in improved oxide uniformity. Stated differently, the silicon carbide layers
and the source oxide wafers may be arranged such that the major surfaces of respective
source oxide wafers are oriented parallel to the silicon carbide layers, such that
respective points on the surface of a silicon carbide layer are located equidistant
from respective points on the major surface of an adjacent source oxide wafer.
[0021] Further embodiments of the invention include forming an oxide layer on a SiC layer
according to conventional techniques and annealing the formed oxide layer in an ambient
containing atomic oxygen. For example, methods of forming an oxide layer on a silicon
carbide layer according to some embodiments of the invention include forming an oxide
layer on a silicon carbide layer, placing the silicon carbide layer with the oxide
layer formed thereon in a chamber substantially free of metallic impurities; heating
an atmosphere of the chamber to a temperature of about 500°C to about 1300°C; introducing
atomic oxygen in the chamber, and flowing the atomic oxygen over a surface of the
silicon carbide layer with the oxide layer formed thereon. The oxide layer may be
formed by a thermal process and/or a deposition process.
Introducing atomic oxygen may include providing a source oxide in the chamber and
flowing a mixture of nitrogen and oxygen gas over the source oxide. The source oxide
may include aluminum oxide or another oxide such as manganese oxide. In some embodiments,
the source oxide is substantially free of metallic impurities. For example, the source
oxide may include a porous sapphire wafer. In such case, some embodiments according
to the invention include implanting a sapphire wafer with non-metallic impurities
to form a porous sapphire wafer, and placing the porous sapphire wafer in the chamber.
CLAUSES SETTING OUT FURTHER ASPECTS AND EMBODIMENTS
[0022]
- 1. A method of forming an oxide layer on a silicon carbide layer, comprising:
placing a silicon carbide layer in a chamber substantially free of metallic impurities;
heating an atmosphere of the chamber to a temperature of about 500°C to about 1300°C;
introducing atomic oxygen in the chamber; and flowing the atomic oxygen over a surface
of the silicon carbide layer to thereby form an oxide layer on the silicon carbide
layer.
- 2. The method of clause 1, wherein introducing atomic oxygen comprises:
providing a source oxide in the chamber; and flowing a mixture of nitrogen and oxygen
gas over the source oxide.
- 3. The method of clause 2, wherein the source oxide comprises aluminum oxide.
- 4. The method of clause 2, wherein the source oxide comprises manganese oxide.
- 5. The method of clause 2, wherein the source oxide is substantially free of metallic
impurities.
- 6. The method of clause 2, wherein the source oxide comprises a porous sapphire wafer.
- 7. The method of clause 1, further comprising: implanting a sapphire wafer with non-metallic
impurities to form a porous sapphire wafer; and placing the porous sapphire wafer
in the chamber.
- 8. The method of clause 1, wherein introducing atomic oxygen comprises generating
atomic oxygen using a catalyst.
- 9. The method of clause 8, wherein the catalyst comprises platinum.
- 10. The method of clause 1, wherein introducing atomic oxygen comprises generating
a flow of ozone; and cracking the ozone to produce atomic oxygen.
- 11. The method of clause 1, wherein introducing atomic oxygen comprises:
generating atomic oxygen outside the chamber; and supplying the generated atomic oxygen
to the chamber.
- 12. The method of clause 1, wherein the chamber comprises an oxidation furnace tube.
- 13. The method of clause 1, wherein heating an atmosphere of the chamber comprises
heating an atmosphere of the chamber to a temperature of about 1000°C to about 1100°C.
- 14. A method of forming an oxide layer on a silicon carbide layer, comprising:
placing a silicon carbide layer in a chamber; placing an alumina wafer in the chamber;
heating an atmosphere of the chamber to a temperature of about 500°C to about 1300°C;
flowing nitrogen gas over the alumina wafer to generate atomic oxygen in the chamber;
and flowing the atomic oxygen over a surface of the silicon carbide layer to thereby
form an oxide layer on the silicon carbide layer.
- 15. The method of clause 14, wherein heating an atmosphere of the chamber comprises
heating an atmosphere of the chamber to a temperature of about 1000°C to about 1100°C.
- 16. The method of clause 14, further comprising flowing oxygen gas over the alumina
wafer.
- 17. The method of clause 16, further comprising: reacting the atomic oxygen with oxygen
gas to produce ozone; flowing the ozone over the alumina wafer; and cracking the ozone
to produce atomic oxygen in the vicinity of the silicon carbide layer.
- 18. A method of forming an oxide layer on a silicon carbide layer, comprising:
placing a silicon carbide layer in a chamber; placing an alumina wafer in the chamber;
heating an atmosphere of the chamber to a temperature of about 500°C to about 1300°C;
nitriding the alumina wafer to liberate atomic oxygen; and flowing the atomic oxygen
over a surface of the silicon carbide layer to form an oxide layer on the silicon
carbide layer.
- 19. The method of clause 18, wherein heating an atmosphere of the chamber comprises
heating an atmosphere of the chamber to a temperature of about 1000°C to about 1100°C.
- 20. The method of clause 18, further comprising flowing oxygen gas over the alumina
wafer.
- 21. The method of clause 20, further comprising: reacting the atomic oxygen with oxygen
gas to produce ozone; flowing the ozone over the alumina wafer; and cracking the ozone
to produce atomic oxygen in the vicinity of the silicon carbide layer.
- 22. The method of clause 18, wherein a major surface of an alumina wafer is parallel
to a major surface of the silicon carbide layer.
- 23. A method of forming an oxide layer on a silicon carbide layer, comprising:
forming an oxide layer on a silicon carbide layer; placing the silicon carbide layer
with the oxide layer thereon in a chamber substantially free of metallic impurities;
heating an atmosphere of the chamber to a temperature of about 500°C to about 1300°C;
introducing atomic oxygen in the chamber; and flowing the atomic oxygen over a surface
of the silicon carbide layer.
- 24. The method of clause 23, wherein introducing atomic oxygen comprises:
providing a source oxide in the chamber; and flowing a mixture of nitrogen and oxygen
gas over the source oxide.
- 25. The method of clause 24, wherein the source oxide comprises aluminum oxide.
- 26. The method of clause 24, wherein the source oxide comprises manganese oxide.
- 27. The method of clause 24, wherein the source oxide is substantially free of metallic
impurities.
- 28. The method of clause 24, wherein the source oxide comprises a porous sapphire
wafer.
- 29. The method of clause 23, further comprising: implanting a sapphire wafer with
non-metallic impurities to form a porous sapphire wafer; and placing the porous sapphire
wafer in the chamber.
- 30. The method of clause 23, wherein introducing atomic oxygen comprises generating
atomic oxygen using a catalyst.
- 31. The method of clause 30, wherein the catalyst comprises platinum.
- 32. The method of clause 23, wherein introducing atomic oxygen comprises generating
a flow of ozone; and cracking the ozone to produce atomic oxygen.
- 33. The method of clause 23, wherein introducing atomic oxygen comprises:
generating atomic oxygen outside the chamber; and supplying the generated atomic oxygen
to the chamber.
- 34. The method of clause 23, wherein the chamber comprises an oxidation furnace tube.
- 35. The method of clause 23, wherein heating an atmosphere of the chamber comprises
heating an atmosphere of the chamber to a temperature of about 1000°C to about 1100°C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023]
Figures 1A - 1C are flowcharts illustrating processing steps for forming oxide layers
on silicon carbide layers according to embodiments of the present invention;
Figure 2 is a schematic illustration of a furnace tube suitable for use in embodiments
of the present invention;
Figure 3 is a schematic illustration of a configuration of silicon carbide wafers
in a furnace tube suitable for use in embodiments of the present invention;
Figure 4A is a graph of capacitance versus voltage for capacitors having an oxide
formed in accordance with some embodiments of the invention;
Figure 4B is a graph of capacitance versus voltage for capacitors having an oxide
formed in accordance with some conventional techniques;
Figure 5 is a graph of capacitance versus voltage for capacitors having an oxide formed
in accordance with some embodiments of the invention as well as capacitors having
an oxide formed in accordance with some conventional techniques;
Figure 6 is a graph of DIT versus energy level from the conduction band for capacitors having an oxide formed
in accordance with embodiments of invention as well as capacitors having an oxide
formed in accordance with some conventional techniques;
[0024] Figure 7 is a graph of capacitance versus voltage for capacitors having an oxide formed in
accordance with some embodiments of the invention;
[0025] Figure 8 is a graph of D
IT versus energy level from the conduction band for capacitors having an oxide formed
in accordance with some embodiments of the invention;
[0026] Figures 9 and 10 are a schematic illustrations of configurations of silicon carbide wafers in a furnace
tube suitable for use in further embodiments of the present invention; and
[0027] Figure 11 is a flowchart illustrating processing steps for forming oxide layers on silicon
carbide layers according to embodiments of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS ACCORDING TO THE INVENTION
[0028] The present invention now will be described more fully hereinafter with reference
to the accompanying drawings, in which preferred embodiments of the invention are
shown. This invention may, however, be embodied in many different forms and should
not be construed as limited to the embodiments set forth herein; rather, these embodiments
are provided so that this disclosure will be thorough and complete, and will fully
convey the scope of the invention to those skilled in the art. In the drawings, the
thickness of layers and regions are exaggerated for clarity. Like numbers refer to
like elements throughout.
[0029] As used herein, the term "and/or" includes any and all combinations of one or more
of the associated listed items. It will be understood that, although the terms first,
second, third etc. may be used herein to describe various elements, components, regions,
materials, layers and/or sections, these elements, components, regions, layers and/or
sections should not be limited by these terms. These terms are only used to distinguish
one element, component, region, layer, material or section from another element, component,
region, layer, material or section. Thus, a first element, component, region, layer,
material or section discussed below could be termed a second element, component, region,
layer, material or section without departing from the teachings of the present invention.
[0030] The terminology used herein is for the purpose of describing particular embodiments
only and is not intended to be limiting of the invention. As used herein, the singular
forms "a", "an" and "the" are intended to include the plural forms as well, unless
the context clearly indicates otherwise. It will be further understood that the terms
"includes", "including", "comprises" and/or "comprising," when used in this specification,
specify the presence of stated features, integers, steps, operations, elements, and/or
components, but do not preclude the presence or addition of one or more other features,
integers, steps, operations, elements, components, and/or groups thereof. As used
herein, the common abbreviation "e.g.", which derives from the Latin phrase "exempli
gratia," may be used to introduce or specify a general example or examples of a previously
mentioned item, and is not intended to be limiting of such item. If used herein, the
common abbreviation "i.e.", which derives from the Latin phrase "id est," may be used
to specify a particular item from a more general recitation.
[0031] Unless otherwise defined, all terms (including technical and scientific terms) used
herein have the same meaning as commonly understood by one of ordinary skill in the
art to which this invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be interpreted as having a
meaning that is consistent with their meaning in the context of this specification
and the relevant art and will not be interpreted in an idealized or overly formal
sense unless expressly so defined herein.
[0032] Embodiments of the present invention provide methods which may enable the formation
of an oxide layer on a SiC layer having an improved interface. These methods may be
especially advantageous in the fabrication of Metal-Oxide-Semiconductor (MOS) devices
created on SiC layers. Using embodiments of the present invention, interface states
with energy levels near the conduction band of SiC may be dramatically reduced. Reduction
of such defects may be advantageous, because these defects may limit a MOS device's
effective surface channel mobility. In addition, the flatband voltage of MOS devices
(i.e. the voltage at which no band bending occurs in the device) may be reduced using
embodiments of the invention.
[0033] Thermal oxidation involves the growth of an SiO
2 layer on a silicon-containing semiconductor layer. As with the thermal oxidation
of Si, in thermal oxidation of SiC, a portion of the underlying semiconductor layer
is "consumed" by the growing oxide layer. As the layer grows, oxygen may diffuse through
the growing oxide layer and react with Si at the SiC surface to form new SiO
2 molecules. Thus, the growth interface advances into the SiC layer as the SiO
2 layer is grown.
[0034] Conventional oxidation of SiC with molecular oxygen (either O
2 or H
2O) to produce MOS-quality Si-SiO2 interfaces has been hampered by slow oxidation rates
(i.e. the rate of growth of the oxide layer) and poor interface quality. Both of these
shortcomings may be attributed to sub-oxide formation due to the transition from SiC
to stoichiometric SiO
2 at the SiC-SiO
2 interface. The oxidation rate may be increased by performing the oxidation at a high
temperature (e.g. 1200 °C or greater). However, high temperature oxidation may be
expensive and may cause unwanted impurities to be introduced into the growing SiO
2 layer, which may result in the presence of unwanted fixed oxide charges in the SiO
2 layer. Some of the interface disorder that may result from oxidation using molecular
oxygen may be passivated by annealing the oxide in a nitrogen containing atmosphere
(NO, N
2O, and/or NH
3) as described above (a so-called "nitridation" anneal). However, such annealing may
not completely passivate the interface disorder.
[0035] Some embodiments of the invention provide methods for oxidizing a SiC layer using
atomic oxygen. Atomic oxygen may exhibit both increased reactivity at the SiC surface
as well as increased mobility within the growing SiO
2 layer. Thus, the use of atomic oxygen to oxidize a SiC layer may result in an interface
having reduced crystallographic disorder. In addition, the oxidation rate may increase
as compared to oxidation using molecular oxygen. Oxidation using atomic oxygen may
moreover be accomplished at lower temperatures than oxidation using molecular oxygen,
which may result in the incorporation of fewer impurities in the oxide from the furnace
ambient.
[0036] In addition, oxidizing a SiC layer using atomic oxygen may reduce the resulting interface
disorder at the SiC-SiO
2 interface, and therefore reduce the density of interface states at the SiC-SiO
2 interface.
[0037] Embodiments of the present invention will now be described with reference to
Figures 1A - 1C which are flow charts illustrating operations according to some embodiments of the
present invention which utilize atomic oxygen to grow an oxide layer on a silicon
carbide layer. Turning to
Figure 1A, a silicon carbide layer is placed in a chamber (block
52). The chamber may be a quartz tube capable of withstanding temperatures in excess of
500 °C. Accordingly, the chamber may act as an oxidation furnace. The SiC layer may
be a SiC substrate and/or a SiC epitaxial layer formed on a homo- or heteroepitaxial
substrate. In particular embodiments, the SiC layer is a single crystal, bulk or epitaxial
layer of 4H-SiC and/or 6H-SiC. In some embodiments, the chamber is configured to accommodate
multiple SiC layers. For example, the chamber may be configured to receive multiple
SiC wafers and/or multiple wafers on which SiC layers are formed. The chamber may
be heated to a temperature of from about 500 °C to about 1300 °C (block
54). In some embodiments, the atmosphere within the chamber may be heated to a temperature
of from about 500 °C to about 1300 °C.
[0038] Next, atomic oxygen is introduced into the chamber (block
56). The atomic oxygen is then flowed over the SiC layer (block
58), resulting in the formation of a SiO
2 layer on the SiC layer.
[0039] As illustrated in
Figure 1B, introducing atomic oxygen in the chamber may include providing a source oxide such
as aluminum oxide within or outside the chamber (block
62) and nitriding the aluminum oxide to liberate atomic oxygen (block
64). The source oxide may be provided in a number of different forms. For example, the
source oxide may include a sapphire wafer or a sintered alumina wafer. In some embodiments,
the source oxide may include a porous sapphire wafer. Polished, non-porous sapphire
wafers have been found not to produce a sufficient amount of atomic oxygen to support
an oxidation process. While not fully understood, it is presently believed that polished,
non-porous sapphire wafers have comparatively little surface area that may be nitrided.
Providing a porous sapphire wafer may provide an increased surface area of aluminum
oxide to be nitrided, which may result in the liberation of a larger quantity of oxygen
than that which may be obtained using a non-porous sapphire wafer.
[0040] Nitriding the aluminum oxide may be performed by flowing nitrogen gas (N
2) over the aluminum oxide at a temperature of from about 500 to about 1300 °C, which
may be sufficient to cause the nitrogen to react with the aluminum oxide, thereby
forming aluminum nitride and liberating a resulting quantity of atomic oxygen. The
process may be self-limiting, since AlN formation at the surface of the aluminum oxide
reduces the atomic oxygen generation process. However, this concern may be mitigated
by increasing the surface area of the aluminum oxide.
[0041] Other methods of introducing atomic oxygen may be employed. For example, molecular
oxygen may be dissociated within the chamber or upstream from the chamber, to provide
atomic oxygen. For example, ozone (O
3) may be provided within the chamber or upstream from the chamber. The ozone may be
dissociated within the chamber due to the elevated temperature within the chamber,
to provide atomic oxygen. Alternatively, molecular oxygen may be dissociated upstream
from the chamber and the resulting atomic oxygen introduced into the chamber. However,
it is presently believed that the mean free path of atomic oxygen is such that, if
produced outside the chamber, at least some of the atomic oxygen would substantially
react with other atomic oxygen before it could be used to grow an oxide layer. Thus,
in some embodiments, the atomic oxygen may be generated within the chamber.
[0042] In some embodiments, atomic oxygen may be generated, e.g. by dissociating molecular
oxygen or by nitriding aluminum oxide, at a location (either within or outside the
chamber) that is spaced away from the SiC layer to be oxidized. For example, in some
embodiments the atomic oxygen may be generated at a location that is farther from
the silicon carbide layer than the mean free path of atomic oxygen in the chamber.
Molecular oxygen gas (O
2) may be provided at a temperature of from about 500 °C to about 1300°C such that
the atomic oxygen may react with the molecular oxygen gas to produce ozone. The resulting
ozone, which may have a larger mean free path than atomic oxygen, may be flowed across
the SiC substrate at a temperature of from about 500 °C to about 1300 °C sufficient
to cause the ozone to dissociate ("crack"), thereby producing atomic oxygen which
may oxidize the SiC layer.
[0043] Figure 1C illustrates methods according to further embodiments of the invention. As illustrated
in Figure
1C, a SiC layer is placed into a chamber (block
70). An alumina wafer is placed into the chamber in proximity to the SiC layer (block
72). The atmosphere within the chamber is heated to a temperature of from 500 °C to 1300
°C (block
74). Next, the alumina wafer may be nitrided to generate atomic oxygen (block
76). For example, nitrogen gas heated to a temperature of from 1000 °C to 1300 °C may
be flowed over the alumina wafer to liberate atomic oxygen from the alumina wafer.
The atomic oxygen is then flowed over the SiC layer to produce a SiO
2 layer on the SiC layer (block
78). In some embodiments, oxygen gas may be flowed over the alumina wafer to form ozone
in the manner described above. The ozone may be flowed across the SiC wafer, where
it may crack, thereby producing atomic oxygen in the vicinity of the SiC layer which
may oxidize the SiC layer.
[0044] Figure 2 is an illustration of a furnace tube suitable for use in particular embodiments of
the present invention. As seen in
Figure 2, the chamber
10, which may be a furnace tube; has a plurality of wafers
12 including SiC layers on which an oxide layer is to be formed. Preferably, the SiC
layers are 4H-SiC. The wafers
12 are placed on a carrier
14 such that the wafers will, typically have a fixed position in the chamber
10. The carrier
14 is positioned so that the wafers are a distance
L1+L2 from an inlet of the chamber
10 and extend for a distance
L3 within the chamber
10. Input gases
16, which may include N
2, O
2, O
3, and/or inert gases, are passed into the chamber
10 and are heated as they traverse the distance
L1 based on a predetermined temperature profile so as to provide the heated gases
18. The heated gases
18 may be maintained at temperatures based on the predetermined temperature profile
and traverse the distance
L2 to reach the first of the wafers
12. The heated gases
18 continue to pass through the chamber
10 until they leave the chamber
10 through an outlet port as exhaust gases
20. Thus, the heated gases
18 traverse the distance
L3. The heated gases
18 are preferably maintained at a substantially:constant temperature for the distances
L2 and
L3, however, as will be appreciated by those of skill in the art in light of the present
disclosure, various temperature profiles may also be utilized. Such profiles may include
variations in temperature over time and/or distance.
[0045] In some embodiments, the SiC layers on the wafers
12 may be oxidized using a predetermined temperature profile which includes an oxidation
temperature of greater than about 500 °C in a chamber in which N
2 and O
2 are supplied at a flow rate profile within predetermined flow rate limits. Preferably,
the temperature of the oxidation is about 1000 °C. The flow rate limits of N
2 and O
2 may be selected based on the particular equipment in which the process is used. However,
in particular embodiments, the flow rate limits of N
2 and O
2 may be as low as about 2 Standard Liters per Minute (SLM) or as high as about 10
SLM or higher. In further embodiments, flow rate limits of about 5 SLM may be preferred.
The oxidation may be carried out for an amount of time dependent on the desired thickness
of the oxide layer. For example, oxidation times of from a few minutes to several
hours or greater may be utilized. In general, oxidation rates are higher for oxidation
using atomic oxygen compared to oxidation using molecular oxygen.
[0046] As noted above, in some embodiments, atomic oxygen may be generated by nitriding
a porous sapphire wafer. Single crystal sapphire wafers are commonly available as
substrates for heteroepitaxial growth of compound semiconductor materials. A porous
sapphire wafer may be formed by ion implantation, for example, by implanting inert
species such as argon and/or nitrogen into the wafer. It may be preferable to use
a porous sapphire wafer instead of alumina as a source of atomic oxygen. As discussed
below, alumina may contain a number of undesirable metallic impurities that may become
incorporated in the oxide. Such impurities may lead to the presence of fixed and/or
mobile oxide charges which can adversely affect the operation of MOS devices. An additional
advantage of using porous sapphire wafers is that after an oxidation process, the
aluminum nitride layer formed on the wafer can be removed and the wafer re-used (possibly
after re-implanting the wafer) as an atomic oxygen source in a subsequent oxidation
process.
[0047] Other oxide materials may be used as a source of atomic oxygen in the manner described
above. For example, manganese oxide may be used instead of aluminum oxide.
[0048] In some embodiments, a catalyst such as platinum may be used to assist and/or encourage
the generation of atomic oxygen. The catalyst may be employed to dissociate molecular
oxygen to generate atomic oxygen and/or ozone upstream from the SiC layer, either
within the chamber or upstream from the chamber. The catalyst is placed between the
SiC layer and the gas source such that when the gas flows over the catalyst, atomic
oxygen is liberated from the gas.
[0049] Other methods of dissociating oxygen may be employed in connection with embodiments
of the invention. For example, atomic oxygen may be formed through the dissociation
of molecular oxygen using optical or electrical energy (e.g. exposure to UV light
and/or electrostatic discharge). In addition, an oxygen plasma may be generated upstream
from the chamber to form atomic oxygen and/or ozone which may be subsequently supplied
to the chamber.
[0050] Figure 3 illustrates particular configurations of SiC wafers and oxides in a chamber 10. As
shown in
Figure 3, in some embodiments, a silicon carbide boat 22 may be placed on a silicon carbide
paddle 20. One or more silicon carbide wafers 24 (which as discussed above may include
bulk SiC wafers and/or wafers on which SiC layers have been formed) may be loaded
onto the boat 22 in a vertical orientation. Alumina blocks or wafers 26 may be provided
on the boat 22 between adjacent SiC wafers 24. The paddle 20 is then placed into a
chamber 10, such as a quartz furnace tube. Nitrogen gas (N
2) and optionally oxygen gas (O
2) are flowed across the alumina blocks 26 and the SiC wafers 24 at a temperature of
from 500 °C to 1300 °C. Atomic oxygen liberated from the alumina blocks 26 oxidizes
the surface of the SiC wafers 24. Although two silicon carbide wafers 24 are illustrated
in
Figure 3, the number of wafers shown in the drawings is arbitrary. It will be appreciated that
the number of wafers that may be processed in a chamber 10 according to embodiments
of the invention will depend on factors such as the size and geometry of the chamber
10.
[0051] Figures 4 to 8 illustrate results which may be obtained utilizing embodiments of the present invention.
Experimental results described herein are provided as examples only and shall not
be viewed as limiting the present invention. Bulk 4H-SiC wafers were placed on a silicon
carbide boat as illustrated in Figure 3. Alumina wafers were positioned between adjacent
SiC wafers, and the boat was placed into an oxidation chamber. The atmosphere of the
chamber was heated to a temperature of 1000 °C. Nitrogen and oxygen gas were flowed
over the alumina substrates and the SiC wafers for 5.5 hours at which time the flow
of oxygen was cut off and the flow of nitrogen was continued for 4 hours, after which
the boat was removed from the chamber. An oxide was observed to have grown on the
SiC wafers, and an aluminum nitride layer was observed to have formed on the alumina
wafers. MOS capacitors were formed at various locations on the oxidized SiC wafers,
and capacitance-voltage (C-V) measurements were taken on the resulting devices. From
the C-V measurements, interface trap density and oxide thickness were calculated.
For comparison, MOS capacitors were formed using conventional molecular oxidation
techniques.
[0052] Figure 4A is graph of measured and theoretical capacitance vs. voltage for a capacitor fabricated
using atomic oxygen according to some embodiments of the invention. As illustrated
in
Figure 4A, the measured capacitance (as indicated by dots
30) was nearly coincident with the theoretical ideal curve
32. Thus, embodiments of the invention may enable the formation of SiC MOSFETs having
very high inversion layer mobility due to reduced interface disorder, thereby producing
power MOSFETs with substantially reduced on-resistance and LDMOSFETs with high frequency
switching capability. In addition, the oxidation rate was observed to double compared
to conventional molecular oxidation.
[0053] For comparison,
Figure 4B illustrates C-V measurements ofMOS capacitor formed using conventional molecular
oxidation. As illustrated in
Figure 4B, the C-V curve of a conventional MOS capacitor (as indicated by dots 34) exhibits
significant deviation from the ideal C-V curve
36.
[0054] Likewise,
Figure 5 is a graph of normalized capacitance (C/C
OX) vs. voltage for a MOS capacitor formed as described above and a MOS capacitor formed
using some conventional molecular oxidation techniques. For the data illustrated in
Figure 5, the conventional MOS capacitor was additionally annealed in a NO environment for
two hours at 1300°C to improve the SiC-SiO
2 interface quality. As illustrated in
Figure 5, the measured capacitance values for the SiO
2 layers formed using atomic oxygen (dots
40) are almost coincident with the ideal curve (line
42), which may indicate that the amount of crystallographic disorder at the SiC-SiO
2 interface is low. The measured capacitance for the NO-annealed SiO
2 layers formed using molecular oxygen (dots
44) shows significant stretch-out as compared to the ideal curve (line
46), indicating that interface traps are present. In particular, the NO-annealed oxide
produced lateral MOSFETs with channel mobility of 50 cm
2/V-s, which is limited by near conduction band states causing a stretch out in the
C-V curve in the flatband to accumulation range. The atomic oxygen C-V data (dots
40) show almost no detectable stretch-out in this region. Assuming negligible interface
trapping, the channel mobility is expected to increase up to 150 cm
2/V-s for lateral MOSFETs fabricated with gate oxides grown in the presence of atomic
oxygen.
[0055] Figure 6 is a graph of interface state density (D
IT) versus position within the conduction band (E
C-E) for SiC-SiO
2 interfaces formed using molecular oxygen and atomic oxygen. As shown in
Figure 6, the interface state density of SiC-SiO
2 interfaces formed using atomic oxygen (dots
45) is significantly reduced compared to that of SiC-SiO
2 interfaces formed using molecular oxygen (dots
47).
[0056] One problem with the use of alumina wafers as a source of atomic oxygen is the presence
of impurities in the wafers. These impurities may become embedded in the oxide and
may result in fixed or mobile charges being present in the oxide which may affect
operation of MOS devices. For example, fixed and/or mobile oxide charge may cause
a voltage shift in the C-V characteristics of the device due to charges moving and
or states trapping an de-trapping. For example, as shown in
Figure 7, a hysteresis loop may be present in the C-V curve
41 as the applied voltage is cycled from high to low and back to high. The presence
of such a hysteresis may indicate a voltage threshold instability in actual devices.
In addition, a MOS interface formed using alumina may have temperature stability concerns.
As illustrated in
Figure 8, after a biased heat treatment of 200 °C, the measured interface trap density D
IT tends to shift upwards (line
51), indicating an increase in the concentration of interface traps from the measurements
taken before heat treatment (line
53). Finally, the oxide thickness of oxide layers grown according to the embodiments illustrated
in connection with
Figure 3 may vary with location on the SiC wafer
24. For example, oxide thicknesses of 450Å were measured at locations on a wafer that
were close to the alumina wafers within the chamber
10, while oxide thicknesses of 300Å, 270Å and 200Å were measured at locations on the
wafer moving sequentially away from the alumina wafers.
[0057] Figure 9 illustrates further configurations of SiC-wafers and oxides in a chamber
10 which may overcome some of the limitations described above. As shown in
Figure 9, a silicon carbide boat
22 may be placed on a silicon carbide paddle 20. One or more silicon carbide wafers
24 may be loaded onto the boat
22 in a vertical orientation. Alumina wafers
28 may be provided on the boat
22 in a vertical orientation between adjacent SiC wafers
24. The paddle
20 is then placed into a chamber
10, such as a quartz furnace tube. Nitrogen gas (N
2) and optionally oxygen gas (O
2) are flowed across the alumina wafers 28 and the SiC wafers
24 at a temperature of from 500 °C to 1300 °C. Atomic oxygen liberated from the alumina
wafers
28 oxidizes the surface of the SiC wafers
24. In these embodiments, since the alumina wafers
28 are oriented in a vertical direction parallel to the orientation of the silicon carbide
wafers such that a substantially uniform distance between the alumina wafers
28 and the SiC wafers
24 is provided, the resulting oxide uniformity may be improved. Stated differently,
the silicon carbide wafers
24 and the alumina wafers
28 are arranged such that the major surfaces of respective alumina wafers
28 are oriented parallel to the silicon carbide wafers
26, such that respective points on the surface of a silicon carbide wafer
26 are located equidistant from respective points on the major surface of an adjacent
alumina wafer
28.
[0058] Figure 10 illustrates further configurations of SiC wafers and oxides in a chamber 10 which
may overcome some of the limitations described above. As shown in
Figure 10, a silicon carbide boat
22 may be placed on a silicon carbide paddle
20. One or more silicon carbide wafers
24 may be loaded onto the boat
22 in a vertical orientation. Porous sapphire wafers
38 may be provided on the boat
22 in a vertical orientation between adjacent SiC wafers
24. The paddle
20 is then placed into a chamber
10, such as a quartz furnace tube. Nitrogen gas (N
2) and optionally oxygen gas (O
2) are flowed across the porous sapphire wafers
38 and the SiC wafers
24 at a temperature of from 500 °C to 1300 °C. Atomic oxygen liberated from the porous
sapphire wafers
38 oxidizes the surface of the SiC wafers
24. In these embodiments, since the porous sapphire wafer may have a high purity, the
chamber
10 may be substantially free of metallic impurities that may become incorporated into
the SiO
2 layer. As used herein, "substantially free of metallic impurities" means that the
resulting SiO
2 layer may have a dose of metallic impurities therein that is about two orders of
magnitude or more lower than the interface state density D
IT of the SiC/SiO
2 interface, i.e. less than about 10
10 cm
-2. Other methods of providing atomic oxygen, such as the dissociation of ozone, may
also result in an oxidation chamber
10 being substantially free of metallic impurities. In addition, since the sapphire
wafers
38 are oriented in a vertical direction such that a constant distance between the sapphire
wafers
38 and the SiC wafers
24 is provided, the resulting oxide uniformity may be improved.
[0059] Further embodiments of the invention include forming a SiO
2 layer on a SiC layer according to conventional techniques and annealing the formed
SiO
2 layer in an ambient containing atomic oxygen. For example, methods of forming an
oxide layer on a silicon carbide layer according to some embodiments of the invention
are illustrated in Figure 9. As shown therein, methods according to some embodiments
of the invention include forming an oxide layer on a silicon carbide layer (block
72), placing the silicon carbide layer with the oxide layer formed thereon in a chamber
substantially free of metallic impurities (block 74); heating an atmosphere of the
chamber to a temperature of about 500 °C to about 1300 °C (block 76); introducing
atomic oxygen in the chamber (block 78), and flowing the atomic oxygen over a surface
of the silicon carbide layer with the oxide layer formed thereon (block 79). The oxide
layer may be formed by a thermal process and/or a deposition process.
[0060] Introducing atomic oxygen may include providing a source oxide in the chamber and
flowing a mixture of nitrogen and oxygen gas over the source oxide. The source oxide
may include aluminum oxide or another oxide such as manganese oxide. In some embodiments,
the source oxide is substantially free of metallic impurities. For example, the source
oxide may include a porous sapphire wafer. In such case, some embodiments according
to the invention include implanting a sapphire wafer with non-metallic impurities
to form a porous sapphire wafer, and placing the porous sapphire wafer in the chamber.
[0061] In some embodiments, introducing atomic oxygen includes generating atomic oxygen
using a catalyst such as platinum. In some embodiments, introducing atomic oxygen
includes generating a flow of ozone and cracking the ozone to produce atomic oxygen.
Ozone may be cracked using thermal and/or electromagnetic energy.
[0062] In some embodiments, atomic oxygen may be generated outside the chamber and supplied
to the chamber.
[0063] In particular embodiments, the atmosphere of the chamber may be heated to a temperature
of about 1000 °C to 1100 °C.
[0064] In the drawings and specification, there have been disclosed typical preferred embodiments
of the invention and, although specific terms are employed, they are used in a generic
and descriptive sense only and not for purposes of limitation, the scope of the invention
being set forth in the following claims.